Three-dimensional profilometry is a common surface profile measuring technology for analyzing the quality of various manufacturing or other kind of processes in the modern time. Generally, the commonly used three-dimensional structured light projection profilometry includes phase-shifting, gray-code, binary-code and random-speckle projecting techniques.
In the phase-shifting profilometry, a multiple phase-shifting such as three-step phase-shifting interferometry is utilized for measuring the object surface by projecting sinusoidal fringe patterns having a series of shifting phases onto the object underlying test, acquiring the object light reflected from the surface of tested object for forming a deformed image, and determining surface high information by analyzing the deformed image through the phase retrieval algorithm. However, in the phase-shifting profilometry, since it is necessary to acquire a plurality of interference images for analyzing the surface profile of the object, it needs time-resolved multiple shifting operation to determine the tested surface of the object, its inspection efficiency is generally not desirable as other one-shot types of surface profiling methods, such as Fourier transform profilometry (FTP), which can only reply on single deformed structured pattern acquisition to complete the phase retrieval.
In the phase shifting profilometry (PSP), as well understood by those skilled in the art, PSP is normally rather precise in surface depth analysis when the tested surface profile varies by incremental steps less than ¼ of the projected period of the structured fringe located on its reference plane between any two adjacent pixels. However, when any surface discontinuity between two adjacent tested pixels is over the above limit exists, the well-known 2π phase ambiguities will be then encountered by PSP to obtain correct profiling result.
Regarding the gray code profilometry, conventionally, it is combined with phase-shifting profilometry (PSP) by projecting structured lights having structural patterns (or fringes) with various gray scale light intensity distribution onto the tested object and acquiring the reflected structured light for forming an object image wherein each pixel on the acquired object image has unique gray scale coding pattern relationship with its neighboring tested pixels. Although the gray scale coding combined with phase-shifting technique can be utilized to detect surface profile without being affected by the above mentioned phase ambiguity, when it comes to increase the depth resolution of surface profile analysis, a series o of multiple structural grey-code patterns with different gray scale variance should be projected onto the surface of object such that the decoding process on the acquired images is normally time-consuming and not competent with one shot measurement.
In the random speckle profilometry using digital imaging correlation (DIC) principle, a structured light having random speckle patterns is projected onto an object underlying test, and a deformed image with respect to the surface of the object is acquired to determine the surface profile of the tested object, wherein a plurality of image blocks having unique deformed (or spatially shifting) random speckle patterns with respect to the surface area of the object are acquired to mathematically correlate with a plurality of image samples stored in a database, which is established through a depth calibration procedure to record various corresponding patterns according to the calibrated depth, thereby obtaining an absolute phase information with respect to the surface depth of the tested object. Since, in the random speckle profilometry, it is often to use small lens apertures to obtain a high depth of field (DOF) measurement so as to acquire the absolute phase information with respect to the object surface. However the depth resolution and accuracy is reduced accordingly due to its large depth measuring range.
To solve the above mentioned problems encountered by the foregoing surface profilometry methods, conventionally, a structured light synthesized by two different periods of projected fringe patterns is utilized to increase the projecting fringe period, so called the equivalent fringe period, which is normally larger than two individual fringe periods. However, the effectiveness is not significant since the vertical measuring resolution is trade off with the measurable step height size. For example, in the technical disclosure “Three-dimensional vision from a multisensing mechanism, Jindong Tian and Xiang Peng, 1 May 2006/Vol. 45, No. 13/APPLICED OPTICS”, a combination including a point-array encoding based on affine transformation and fringe encoding based on phase mapping is utilized to detect three-dimensional object surface having arbitrary geometric shapes, wherein the point-array encoding is initially applied to determine the fringe orders to create a control vertex mesh with absolute coordinate values in 3D space while the phase evaluation and phase unwrapping for fringe decoding is performed under the guidance of control vertex mesh. Since Tian disclosed two specific structured lights belonging to non-random structured light, the phase ambiguity may still be encountered when large surface discontinuity exists on the surface of the tested object. This is simply because the detected depth information is not based on the absolute phase basis.
Accordingly, there still is key need to provide a measuring apparatus and method for three-dimensional profilometry of an object for improving the above mentioned disadvantages of the conventional measuring technologies for surface profilometry.